Japanese Patent Laying-Open No. 10-290529 discloses a power supply device incorporated in an electric vehicle. The power supply device includes a battery, an electric circuit system such as a running motor and an on-vehicle auxiliary machine supplied with power from the battery, an inverter circuit for generating a commercial alternating-current (AC) voltage that converts a direct-current (DC) voltage from the battery to a commercial AC voltage for application to a commercial power supply load, a cutoff switch provided between the inverter circuit and the commercial power supply load, and a leakage detecting circuit that detects a ground-fault current leaking from the battery so as to detect leakage of the electric circuit system.
In this power supply device, upon detection of the leakage, the leakage detecting circuit stops the inverter circuit and causes the cutoff switch to operate to cut off the power feeding to the commercial power supply load, without interrupting the power feeding to the electric circuit system such as the running motor and the on-vehicle auxiliary machine.
Due to the effect of the capacitive component of the commercial power supply load, the impedance varies between the case where the commercial power supply load is electrically connected to the power supply device and the case where the commercial power supply load is not connected to the power supply device. In the above-mentioned Japanese Patent Laying-Open No. 10-290529, however, the variations in impedance due to the effect of the capacitive component of the commercial power supply load is not taken into consideration. Thus, the decrease in insulation resistance of the power supply device cannot be accurately detected.